In mobile communications systems, mobile stations and base transceiver stations may set up connections through channels of a so-called radio interface. Depending on the type of information to be transferred, demands are made on the connections in regard to faultlessness of transferred data and in regard to transfer lag.
A certain frequency area is always allocated for use by the mobile communications system. To have sufficient capacity in the mobile communications system on this limited frequency band, the channels which are in use must be used several times. For this reason, the coverage area of the system is divided into cells formed by the radio coverage areas of individual base transceiver stations, which is why the systems are often also called cellular radio systems.
FIG. 1 shows the main structural features of a known mobile communications system. The network comprises several inter-connected MSCs (Mobile Services Switching Centre). The mobile services switching centre MSC can set up connections with other mobile services switching centres MSC or with other telecommunication networks, e.g. ISDN (Integrated Services Digital Network), PSTN (Public Switched Telephone Network), Internet, PDN (Packet Data Network), ATM (Asynchronous Transfer Mode) or GPRS (General Packet Radio Service). Several base station controllers BSC are connected to the mobile services switching centre MSC. Base transceiver stations BTS are connected to each base station controller. The base transceiver station may set up connections with mobile stations MS. A network management system NMS may be used for collecting information from the network and for changing the programming of network elements.
The air interface between base transceiver stations and mobile stations can be divided into channels in several different ways. Known methods are at least TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) and CDM (Code Division Multiplexing). The band available in a TDM system is divided into successive time slots. A certain number of successive time slots forms a periodically repeating time frame. The channel is defined by the time slot used in the time frame. In FDM systems, the channel is defined by the used frequency, while in CDM systems it is defined by the used frequency hopping pattern or hash code. Combinations of the division methods mentioned above can also be used.
FIG. 2 shows an example of a known FDM/TDM division. In the figure, frequency is on the vertical axis while time is on the horizontal axis. The available frequency spectrum is divided into six frequencies F1-F6. In addition, the frequency channel formed by each frequency is divided into repeating time frames formed by 16 successive time slots. The channel is always defined by the couple (F, TS) of frequency F and time slot TS used in the time frame.
In order to maximise capacity, channels must be reused in cells which are as close to one another as possible, however, so that the quality of connections using the channels will remain sufficiently good. The connection quality is affected by the sensitivity of transferred information to transfer errors occurring on the radio channel and by the quality of the radio channel. The transfer error tolerance of the signal depends on the characteristics of the transferred information and it can be improved by processing the information with channel coding and interleaving before sending it to the channel and by using retransmission of faulty transfer frames. The Carrier to Interference Ratio (CIR) depicts the radio channel quality which ratio is the ratio between the strengths of the signal sent by the sender and perceived by the recipient on the one hand and of the interference caused to the channel by other connections on the other hand.
FIG. 3 shows the emergence of interference caused to each others by simultaneous connections. In the figure three mobile stations MS1, MS2 and MS3 communicate with base transceiver stations BTS1, BTS2 and BTS3. The signal received by base transceiver station BTS1 contains a signal S1, which is sent by mobile station MS1 and which is showed by a solid line and the power of which depends on the transmission power used by mobile station MS1 and on fading on the radio path between mobile station MS1 and base transceiver station BTS1. Typically, the radio path fading is smaller with a shorter distance between base transceiver station and mobile station. In addition to signal S1, the signal received by the base transceiver station contains signal components I21 and I31 caused by signals sent by mobile stations MS2 and MS3. Components I21 and I31 will cause interference in the reception, if they can not be filtered away from the signal received by the base transceiver station. Correspondingly, the signal sent by mobile station MS1 causes signal components I12 and I13 in the signals received by base transceiver stations BTS2 and BTS3 and these signal components may cause interference in the receptions. Components of a similar kind also emerge in the signals received by mobile stations from base transceiver stations.
If signal components I21 and I31 are on the same channel as signal S1, they can not be removed by filtering. Interference may also be caused by signals occurring on other channels than on the same channel. E.g. in systems using FDM frequency division, channels which are adjacent to one another on the frequency level are always slightly overlapping in order to use the frequency spectrum as effectively as possible, which will result in reception interference also from signals which are on the adjacent channel. Correspondingly, when using code division CDM, connections using codes that are too much alike will cause interference to one another. However, so-called neighbour channel interference caused by signals on other channels is considerably smaller than the interference caused by equally powerful signals on the same channel.
The magnitude of interference caused by connections to each other thus depends on the channels used by the connections, on the geographical location of connections and on the transmission power used. These may be influenced through a systematic allocation of channels to different cells taking the interference into account, through transmission power control and through averaging of the interference experienced by the different connections.
It is an objective in channel allocation to allocate such channels to the desired connections which may all be used at the same time while the signal quality remains acceptable. To maximise capacity, channels should be reused as close to one another as possible. The distance at which one and the same channel can be reused so that the CIR remains acceptable, is called the interference distance while the distance at which one and the same channel is reused is called the reuse distance.
Known methods of channel allocation are Fixed Channel Allocation (FCA), Dynamic Channel Allocation (DCA) and Hybrid Channel Allocation (HCA) which is obtained as a combination of FCA and DCA. The idea in fixed channel allocation is to divide the channels used in the system between the cells through a frequency design which is made before the system is put into use. In dynamic channel allocation, all channels are in a common channel pool, from which for the connection to be set up the best channel is chosen for use according to some predetermined norm. In hybrid channel allocation, some of the channels used in the system are divided in a FCA fashion fixedly for use by different cells and the remaining channels are placed in a channel pool, from which they may be taken as required dynamically for use by all cells. The different methods are described very thoroughly in the publication I. Katzela and M. Naghshineh: "Channel Assignment Schemes for Cellular Mobile Telecommunication Systems: A Comprehensive Survey", IEEE Personal Communications, pp. 10-31, June 1996.
Dynamic channel allocation methods can be divided into centralised and decentralised methods. Decentralised methods, wherein the channels are allocated independently in each cell, can be divided further into methods based on the channel allocation situation, on knowledge of radio path fading and on measurement of the occurrence of interference on the channel. In decentralised methods based on knowledge of the allocation situation, information on that allocation situation of channels must be maintained for each base transceiver station which affects the allocation of channels of the base transceiver station's cell. The problem then is the high quantity of signalling. In methods based on measurement of the occurrence of interference on the channel, the best channel for the connection to be set up is determined by measuring the interference level of channels and by, according to the measurements, choosing the channel allowing a sufficiently good carrier to interference ratio. Measurements can never be entirely in real time. For this reason, the method suffers from the lag in measurement data used in the making of allocation decisions, especially in communications containing a lot of short and burst-like transmissions. The centralised methods, wherein the allocation of channels for several cells is done in a centralised fashion, are based on a knowledge of the channel allocation situation and of radio path fading. With this method an almost optimum channel allocation can be achieved, but the high quantity of necessary computing is a problem especially in larger systems.
Using a carrier to interference ratio CIR which is higher than necessary will hardly improve the connection quality in digital systems but will just unnecessarily increase the interference caused to other connections. The difference between the carrier to interference ratio CIR(min) required by the connection and the carrier to interference ratio CIR which can be achieved on the radio channel at the transmitter's maximum transmission power will be called the carrier to interference margin CIRM=CIR-CIR(min) hereinafter. The carrier to interference margin can be used to achieve a carrier to interference ratio which is higher than what the connection requires and/or to lower the transmission power. By lowering the transmission power the interference caused to other connections is reduced at the same time. It is in fact possible considerably to reduce the channel interference distance and this way also the reuse distance by controlling dynamically the transmission power used by connections. A reduction of the reuse distance again will add to the system's capacity. A dynamic control of the transmission power aims at maintaining an adequate connection quality, however, at the same time minimising the transmission power used. Interference may also be reduced e.g. by using directional antennas, whereby the same carrier to interference ratio can be achieved with a lower transmission power.
Different connections experience different interference even after a very successful channel allocation. Some connections may hereby suffer from an interference limiting the connection quality even considerably while other connections would at the same time tolerate an even higher interference level. A channel may be allocated, if the carrier to interference ratio achieved by the connections set up on the channel is below a certain CIR(min) limit for just a small part, e.g. for 5 percent of the connections set up. If variations in the interference level between different connections can be reduced, then the said connection quality requirement can be achieved with an even denser reuse of channels, which will increase system capacity. The situation is explained in FIG. 4, wherein the interference caused to the channel in relative units is on the horizontal axis while the probability of its occurrence for two different interference distributions interference 1 and interference 2 is on the vertical axis. The requirement is that occurrence of interference on the channel is less than 75 units for 95 percent of the connections. Since the distribution of interference 1 is broad, its mean value must be set at the point 50 units for meeting the requirement. Correspondingly, the distribution of interference 2 is considerably narrower, whereby its mean value may be set at the point 70 units. Thus, the average interference may be the higher the less variation of interference there is between the different connections. Correspondingly, the required average interference will determine the reuse distance of channels. By reducing the variation of interference between connections it is thus possible to make denser the reuse of connections and thus to increase network capacity.
Known methods of equalising interference between different connections are frequency hopping in FDM systems and time slot hopping in TDM systems. The name of channel hopping method will be used hereinafter in this application for the methods mentioned above and for other methods based on changing of channel. In CDM systems, interference between connections is equalised through the use of hash codes which are sufficiently different. On the other hand, all connections use the same frequency in the method, which considerably increases the average value of mutual interference.
In frequency hopping, the frequency of the connection is changed frequently. The methods may be divided into quick and slow frequency hopping respectively. In quick frequency hopping, the connection frequency is changed more frequently than the frequency of the used carrier frequency. In slow frequency hopping again the connection frequency is changed more seldom than the frequency of the used carrier frequency.
E.g. in the known GSM system, frequency hopping is carried out so that the individual burst is always sent at one frequency while the burst sent in the next time slot is sent at another frequency. The individual burst may hereby suffer even from a high interference level. However, owing to channel coding and interleaving the connection will be of a sufficiently good quality if a sufficient number of bursts can be transmitted without significant interference. With the aid of frequency hopping this condition is fulfilled for individual connections, even if some bursts would suffer from quite bad interference.
A frequency hopping arrangement is shown in FIG. 5 which illustrates frequencies used with different bursts. Six frequencies, frequencies F1-F6, are allocated for use in the cell. The hopping pattern is cyclic in such a way that listed from the start of the cycle the cell sends its bursts at frequencies F6, F2, F5, F1, F4, F6, F3, F5, F2, F4, F1, whereupon the cycle is repeated. Since the cycle length is 11 bursts, the individual connection in a system e.g. according to FIG. 2 using time frames of 8 time slots will use the same frequency in approximately every fifth burst. Hereby the different fadings experienced at different frequencies by a connection between mobile station and base transceiver station will also be well averaged. The best results in terms of interference equalisation are achieved with frequency hopping when the frequency hopping patterns used in cells located near one another are independent of one another. This is achieved by using carefully chosen periodic or pseudo random frequency hopping patterns.
Time slot hopping is quite similar to frequency hopping in principle. In time slot hopping, the time slot used in the connection is exchanged instead of the frequency. In time slot hopping too the hopping patterns used must be independent of each other in cells located close to one another in order to achieve the best result.
As the number of mobile station subscribers is growing and applications demanding a big band width, such as multimedia applications, are becoming more usual, state-of-the-art methods of channel allocation are no longer able to utilise the available frequency spectrum with sufficient efficiency. Special problems are caused by situations where a limited frequency band is used jointly by several different systems, for example, by a mobile communications system and a wireless office system. It is an objective of the present invention to alleviate these problems by making channel allocation even more effective. This objective is achieved with the method described in the independent claims.